Modulation system



Dec. l, 1942. J. EvANs MODULATION SYSTEM Filed May 27, i941 attorney Y from the practical standpoint,

Patented ec. l, 1942 STATES PATENT OFFICE 2,303,444 MODULATION SYSTEM John Evans, Palmyra, N. J.,

assignor to Radio ware Application May 27,

6 Claims.

lThis invention relates to a modulation system and particularly to a system of -phase or 1941, Serial N0. 395,329

frequency modulating a radio frequency carrier and for stabilizing the mid-frequency of the carrier.

An amplitude modulation system may be definedas one in which the amplitude of the carrier is varied as a function of the signal so that the envelope of the carrier represents the signal. In phase modulation, if a signal of constant amplitude and variable frequency is applied to a carrier, sidebands of constant amplitude are established for all signal frequencies. If a similar signal of constant amplitude and variable frequency is applied to frequency modulate a carrier, the amplitude of the resulting sidebands will be different for different audio frequency. inasmuch as the output from a frequency modulated transmitter may be converted, by means of a signal modifying network, into a phase modulated wave, or a phase modulated transmitter into a frequency modulated wave, no distinction will be made in describing this invention which may be applied to frequency or phase modulating devices. Therefore, it should be understood that the terms phase modulation and frequency modulation are used synony-mously.

In a copending application modulated transmitters, Serial No. 390,053, filed April 24, 1941, by John L. Barnes and assigned as the instant application, a frequency or phase modulation system isA described. While the mid-frequency is stabilized, a substantial number of filter sections are required to, obtain the desired phase shift. application discloses a system or method for btainng the desired phase shift in a transmission line, which may readily be made of the required length by using high frequency and hence short wave lengths. If currents of high frequency are applied to a line, of say m length where n is a constant and i is the Wave length of the applied current, the phase shift will be multiplied in the line by n. It is a decided improvement, especially to use a line in place of a multisection lter. The line is not only cheaper but any required coiled into a small space.

The invention will be described -by referring to the accompanying drawing in which Figure 1 is a schematic block diagram of one embodiment of the invention; and Figure 2 is a graph illustrating the phase shift ofthe invention.

Referring to Fig. 1, a crystal oscillator l or other stabilized source of high frequency oscilla.-

The instant entitled Phase l length may be tions is connected through a frequency multiplier 3 to a transmission line 5 The output of the transmissionvline is connected toa beat frequency detector 1. The output of the beat `frequency detector yis applied to a transmitter 9 which is connected to an antenna Il or other load circuit. The signal is applied, through a pre-emphasis network I3 is desired, to a reactance modulator l5. The reactance modulator I5 is connected to a' frequency modulated, and preferably stabilized, oscillator I1. The output of the frequency modulated oscillator is applied to the transmission line 5.'

The operation of the circuit is follows: The currents from the crystal oscillator are multiplied to obtain a stabilized source of current of a frequency of 200 megacycles, by way of example. These currents are shifted in phase by the transmission line 5 but the phase shift is constant at the output of the line because the frequency of the applied current is constant. In the absence of modulation, currents of say 150 megacycles, from the line stabilized oscillator willy also be subjected to a constant phase shift in the transmission line. The currents from the two sources 3 and `I1 are combined in the beat detector to produce a beat frequency of 50 megacycles which is applied through the transmitter to the load circuit. Since both sources are stabilized the output currents will be stabilized at the mid or unmodulated frequency Fo.

If the signals are applied the reactance modulator l5 will react on the line stabilized oscillator to change its frequenc The new frequency, say F+ will be shifted in phase as indicated in Fig. 2. If the line 5 is many times longer than the wave length the phase will be shifted by many radians. For example, if the reactance tube shifts the phase by one radian and the phase shifting transmission line is the phase will be shifted 100 radians. shifted for an increasing frequency in Fig. 2. Thus the phase isvadvanced or retarded with respect to the phase of the midfrequency and the currents applied to the load circuit are phase or frequency modulated.

While the two frequencies have been selected as and 200 megacycles, it should be understood that these were chosen by way of exemplication and not'limitation. Furthermore, the transmitter may include a frequency multiplier to multiply the output frequency if a greater frequency shift than may be conveniently obtained by the transmission line 5 is required. Inasquency oscillator, and

' frequency stabilized much as the oscillators, reactance modulator, frequency multiplier, transmission line,'beat frethe transmitter are all elements well knownto those skilled in the art.

detailed wiring diagrams are unnecessary to an l understanding of the invention.

I claim as my invention:

1. A modulation system including a source 'of frequency stabilized oscillations, a sounce of phase modulated oscillations, a transmission line for shifting the phase of said modulated oscillations, means for applying oscillations from said two sources to the input of said line, means connected tosaid line for combining the oscillations applied' thereto to obtain currents having a frequency equal to the beat frequency of said applied oscillations, and means for applying said4 beat frequency currents to a load circuit;

2. A modulation system including a source of oscillations, means connected to said source for multiplying said frequency, a source of phase modulatedoscillations, a transmission line of a length equal to 11A where n4 equals a constant and A equals the wave length of said phase modulated oscillations, means for applying oscillations from said source and4 said multiplier means to said line, a beat frequency detector effectively connected tofsaid line for deriving a current corresponding to the beat frequency of the applied currents, and means for applying said beat frequency currents to a load rent to a load circuit.

4. A modulation system including a source of frequency stabilized oscillations, a source of phase modulated oscillations. a transmission line of a length corresponding to nl, where n is a constant and A is the Wave length of the modulated oscil-4 lations, means for applying oscillations from said sources to the input of said line, means associated Withthe output of said line for combining the line output currents to obtain a current corresponding to the difference in frequency of the applied oscillations, and means for applying said difference frequency current to a load circuit.

5. A modulation system including a source of frequency stabilized oscillations, a source of phase modulated oscillations, a transmission line for shifting the phase of said modulated oscillations, means for applying oscillations from said two sources to the inputI of said line, means connected to said line for combining the oscillations applied thereto to obtain currents having a frequency equal to the difference in frequency of said applied os'cillations, and means for applying said difference frequency currents to a load circuit.

6. A modulation system including a source of frequency stabilized currents, means connected to said sourcefor multiplying said frequency, a source of phase modulated currents, a transmission line connected to said multiplying means for shifting the phase of said frequency multiplied currents a constant amount, means for applying said phase shifted currents to said transmission line for multiplying the amount of said phase shift, means effectively connected to said transmission line for combining the currents applied thereto so that the resulting current has a frequency equal to the beat frequency of the applied currents and a phase shift which varies as the phase shift of said phase modulated currents multiplied by the phase shift occurring in said line.

JOHN EVANS. 

